Method and System for the Utilization of an Energy Source of Relatively Low Temperature

ABSTRACT

A method of extracting energy from an external heat source. The method comprises the steps of: a) compressing a medium in the liquid phase using an external power source to obtain a compressed liquid medium; b) heating the compressed liquid medium from step a) using heat at least partly derived from the external heat source to expand the medium and obtain it in the supercritical state; c) reducing the pressure of the heated medium from step b) to a controlled degree by applying a variable load to generate electric power of a frequency; d) converting the frequency of step c) to a desired output frequency; and e) reducing the temperature and volume of the medium from step c) to obtain the medium in the liquid phase for recycling to step a), wherein the degree of compression in step a) is controlled independently of the load applied in step c). A corresponding system is also provided.

TECHNICAL FIELD

The present disclosure relates to energy extraction, e.g. from alow-temperature energy source such as exhaust gas or water. In someembodiments, the invention relates to a closed heat engine systemcontaining a medium, which in a first part of the system is in asub-critical phase and in a second part of the system is in asupercritical phase, and where the system comprises a turbine to whichthe medium is supplied in its super-critical phase.

BACKGROUND

Substances change phases according to the prevailing temperature andpressure. A familiar example is water, which at a pressure of 1 bar is asolid (i.e. ice) at a temperature below 0° C., a liquid at temperaturesbetween 0 and 100° C., and a gas (vapor) when temperature is over 100°C. Heating a substance in a given phase requires a certain amount ofenergy per degree of temperature increase, while considerably moreenergy is required to bring the substance from one phase to another eventhough temperature does not increase. An illustrative example is water.A temperature increase of 1 K (Kelvin) in ice requires 2.05 J/g. Phasetransition from ice to water requires 334 J/g. A temperature increase of1 K in water requires 4.18 J/g. The phase transition from water to steamrequires 2257 J/g. Temperature increase of 1 K in the steam requires2.08 J/g at 100° C.

Phase transition is dependent on ambient temperature and pressure and ischaracteristic for each substance. This is presented visually in a (P,T) phase diagram well known within the art. Each substance is alsocharacterized by its critical temperature and critical pressure, theso-called critical point. When both pressure and temperature are greaterthan their respective critical value, the substance enters a conditionknown as a super-critical fluid. Phase transition between liquid and gasceases and there is no related phase transition energy. In this state,the substance has both liquid and gas characteristics. It behaves like agas as it will fill a container homogeneously, while it weighs onlyslightly less than its own weight in liquid phase. The super-criticalpoint for water is for a pressure of 22.064 MPa (218 atm, 221 Bar) and atemperature of 304° C. In comparison, the critical point for CO₂ is apressure of 7.380 MPa (72.8 atm, 73.8 Bar) and a temperature of 31.04°C.

When a traditional Rankine Cycle comprising a steam turbine is employed,the temperature differential should be as big as possible; both to avoiddrops of water to condense inside the delicate and high-speed rotatingsteam turbine, and to build as much thermo-dynamic energy into the steamas possible. The trend on e.g. concentrated solar power energy plants isto increase the steam temperature, and a temperature of 400-500° C. isoften used. But also lower temperature systems using steam temperaturesas low as 200-300° C. requires a heat source having a higher temperatureto operate.

After the steam has been condensed in the condenser, the temperature isso low compared to the water fed into the boiler that it cannot beefficiently used to heat the inlet water. And it cannot be utilized tobuild temperature into the steam.

Both these effects are contributors to the inherent limitation ofefficiency for the rankine based steam heat engines.

In thermal power plants using either fossil fuels such as coal or oil,or a radioactive source, energy is used to convert water into steam.Steam drives the steam turbine which drives generators to produceelectrical energy. After the steam has flowed through the steam turbine,it is condensed to water and returned to heating. As mentioned above,the phase transition from water to steam requires a significant amountof energy. This same amount of energy is released when steam condensesback to water. In thermal power plants, little of the condensationenergy can be re-used, and a large part of it is instead expelled withcooling water. This constitutes a significant waste of energy.

For refrigeration, air conditioning and heat pumps, a medium thatundergoes a cycle between liquid and gas is used. Besides conventionalrefrigerants such as R-12 and R-22 also known as freon, it is known touse R-134a, ammonia or CO₂ as the refrigerant.

In a traditional heat pump, energy is obtained from a heat source at arather low temperature. The heat pump evaporator is connected to theheat source, and under the pressure and temperature conditionsprevailing in the evaporator, the medium in the heat pump has a boilingtemperature that is lower than the temperature of the heat source. Themedium is supplied with energy in the evaporator. The evaporator is thusthe heat pump energy collector and it is located upstream of the heatpump compressor. The compressor increases temperature and pressure inthe medium vapor phase. In the heat pump condenser, the medium is cooledand goes from gas to liquid phase. There, the condensation heat isreleased to a heat sink in connection with the condenser. As an example,a heat pump heats consumption water or water in water-borne heatingsystems by harnessing energy in air, soil or water. The energyefficiency (or coefficient of performance) of a heat pump, which oftenis a factor of 3-4, depends on the temperature difference between theheat source and the heat sink. The same principle is applied inair-conditioning systems, cooling and freezing systems.

It is known in the field to use CO₂ as refrigerant in heat pumps wherethe CO₂ in parts of the heat pump is in a sub-critical phase and inother parts is in a super-critical phase. This is described as atrans-critical cycle. Patent publications WO90/07683, U.S. Pat. No.6,877,340 and U.S. Pat. No. 7,674,097 describe such heat pumps. Inthese, the compressor is used to bring CO₂ into a supercritical phase.For these heat pumps to work, the CO₂ must be brought to sub-criticalphase before reaching the heat absorber. In the heat absorber, CO₂ isheated, but remains in the subcritical phase. CO₂ is brought tosubcritical phase before the heat absorber by means of a reducing valveas described in WO90/07683, or a so-called expander as described in U.S.Pat. No. 6,877,340 and U.S. Pat. No. 7,674,097. In the expander thevolume is increased from the inlet to the outlet. The medium volumeincreases and the pressure decreases. CO₂ is thus brought to asubcritical phase before reaching the heat absorber. U.S. Pat. No.6,877,340 and U.S. Pat. No. 7,674,097 teach how an expander can beutilized to reduce energy consumption in the heat pump by partly drivingthe compressor by means of a shaft driven by the expander, or by drivingan auxiliary compressor thanks to the expander. U.S. Pat. No. 6,877,340also teaches that the expander can drive a generator to produceelectrical energy.

Energy in solid wastes can be exploited by incineration to produce steamthat can be used for industrial purposes, or to produce electricity inthe same way as in a thermal power plant. Energy can also be used toheat water to be distributed in district heating networks. As is knownfrom shipping, energy from exhaust gases can also be used to heat water.

Electrical energy has long been considered to be the most versatile formof energy, and efforts to turn the energy from wind, waves, tides andsunlight into electricity have been considerable and fruitful. However,there is also considerable energy in the form of media not hot enough toproduce steam, and it would be a great advantage to be able to produceelectricity out of them.

SUMMARY OF THE INVENTION

The inventors have realized that it is important to be able tocontrol/balance the pressure in various positions in/segments of thesystem in order to efficiently convert the heat from a medium ofrelatively low-temperature. That is, the pressure control should beachieved without substantial energy losses.

U.S. Pat. No. 3,971,211 discloses a system in which CO₂ is pressurized,heated with energy from an external source, expanded over two turbinesin series, cooled and returned for pressurization. The CO₂ ispressurized by means of a pump that is driven by the first turbine. Thismeans that degree of compression in the pump is determined by the amountof heat supplied upstream the turbine and the degree of coolingdownstream the turbine. Consequently, the means of controlling thesystem of U.S. Pat. No. 3,971,211 are limited. Further, it will manytimes be impractical, if not impossible, to control the degree ofheating and cooling, respectively in an industrial setting. To run thesystem of U.S. Pat. No. 3,971,211 it is probably necessary to installseveral valves controlling the flow and pressure of the medium, e.g.upstream the respective turbines. Such valves would waste energy andthus decrease the efficiency of the system.

An example of system having such valves is disclosed in US 2009/0266075.In the system, liquid CO₂ is pressurized by a pump and transferred to astorage. Further, CO₂ from the storage is heated with an external heatsource, expanded in an “expansion machine”, liquefied and returned tothe pump. A valve is arranged between the heating and the expansion,probably to control the pressure during the heating. Part of the energytaken up by the medium in the heating step will thus be wasted beforethe turbine. Further, the flow through the system appears to becontrolled by controlling the supply of pressurized CO₂ to the heatingstep from the storage. This requires the use of further valves, whichfurther reduces the net efficiency of the system. Also, the storagerequires bulky equipment which is expensive.

The present disclosure provides a heat engine system for extractingenergy from an external heat source, comprising:

-   -   a pump for compressing a liquid medium to obtain a compressed        liquid medium, said pump comprising an inlet, an outlet and an        external motor capable of controlling the degree of compression        of the liquid medium in the pump;    -   a heating arrangement connected to the external heat source for        heating and expanding the compressed liquid medium to obtain the        medium in the supercritical state, said heating arrangement        comprising an inlet connected to the pump outlet and an outlet;    -   a turbine for generation of mechanical work from the medium from        the heating arrangement, said turbine comprising an inlet        connected to the heating arrangement outlet and an outlet;        a electricity generator connected to the turbine, said        electricity generator being capable of controlling the load of        the turbine such that the pressure upstream of the turbine may        be controlled;    -   a frequency converter connected to the electricity generator;        and    -   a cooling arrangement for reducing the temperature and volume of        the medium, said cooling arrangement comprising an inlet        connected to the turbine outlet and an outlet connected to the        pump inlet,        wherein the compression in the pump is controllable        independently of the load of the turbine.

Further, there is provided a method of extracting energy from anexternal heat source, comprising the steps of:

a) compressing a medium in the liquid phase using an external powersource to obtain a compressed liquid medium;b) heating the compressed liquid medium from step a) using heat at leastpartly derived from the external heat source to expand the medium andobtain it in the supercritical state;c) reducing the pressure of the heated medium from step b) to acontrolled degree by applying a variable load to generate electric powerof a frequency;d) converting the frequency of step c) to a desired output frequency;e) reducing the temperature and volume of the medium from step c) toobtain the medium in the liquid phase for recycling to step a),wherein the degree of compression in step a) is controlled independentlyof the load applied in step c).

In the above system, the pump is not driven by a turbine arranged in thesystem as in U.S. Pat. No. 3,971,211. Instead, the pump is driven by anexternal motor and the degree of compression in the pump may becontrolled independently of the work generated by a turbine in thesystem. The motor of the present disclosure is typically a variablefrequency drive motor capable of driving the pump at a desired speed.Thus, no energy-wasting valves are needed to control the flow to thestep where the medium is heated by the external heat source.

Further, the load in turbine and thereby the pressure drop over theturbine is controlled by an electricity generator connected to theturbine, typically (but not necessarily) via a turbine shaft. The loadmay thus be controlled to obtain a pressure balance in the system thatmatches the heating capacity in the heating step and the coolingcapacity on the cooling step.

For example, the medium is pumped upstream the heating step and the flowof the medium is restricted downstream the heating step such that anoverpressure is obtained in the heating step. In prior art systems, anenergy wasting valve is arranged downstream the heating system torestrict the flow. In the present disclosure, the turbine and generatorarrangement is instead adapted to variably restrict the flow and therebycontrol the pressure in the heating step without wasting energy.

In the system of the present disclosure, it is also possible to restrictthe pressure drop over the turbine such that the pressure of mediumnever falls below its supercritical pressure. Accordingly, it ispossible to reduce the amount of energy consumed by the motor of thepump to reach the pressure desired for the heating step.

A consequence of the variable load in the turbine is that the frequencyof the electricity generated will not always match the desired outputfrequency. Therefore, a frequency converter is connected to theelectricity generator. In contrast, traditional turbines are set to afixed frequency that matches the desired output frequency.

A principal difference between most prior art disclosures and thepresent disclosure is that an expansion of the medium is used togenerate the work producing electricity in the prior art, while thepresent disclosure relies on a pressure reduction of the medium withminimal expansion to generate work and electricity. The uncontrolledexpansion in the prior art disclosures normally results in a temperatureof the medium after the turbine that is so low that its energy cannot berecovered. However, the medium from the turbine of the presentdisclosure may for example be used for heating the pressurized mediumfrom the pump (internal heating) or a medium of a district heatingsystem (external heating).

The system of the present disclosure may be liken to a hydraulic systemwherein the expansion of the medium during the heating is controlled andutilized by the turbine, which has an hydraulic capacity thatcorresponds to volume of the medium after the expansion, and the pumphas an hydraulic capacity that corresponds volume of the medium beforethe expansion.

FIGURES

FIG. 1 shows a schematic piping diagram of a heat engine systemaccording to a first embodiment.

FIG. 2 shows a schematic piping diagram of a heat engine systemaccording to a second embodiment.

FIG. 3 shows a schematic piping diagram of a heat engine systemaccording to a third embodiment.

FIG. 4 shows a phase diagram for carbon dioxide, in which the x-axisrepresents the temperature (K) and the y-axis represents the pressure(bar). The cycle to the left in the figure illustrates the inventivemethod, while the cycle to the right in the figure illustrates analternative configuration of the inventive method.

FIG. 5 shows a schematic piping and control diagram of a heat enginesystem according to a fourth embodiment.

DETAILED DESCRIPTION

As a first aspect of the present invention, there is thus provided amethod of extracting energy from an external heat source, comprising thesteps of:

a) compressing a medium in the liquid phase using an external powersource to obtain a compressed liquid medium;b) heating the compressed liquid medium from step a) using heat at leastpartly derived from the external heat source to expand the medium andobtain it in the supercritical state;c) reducing the pressure of the heated medium from step b) to acontrolled degree by applying a variable load to generate electric powerof a frequency;d) converting the frequency of step c) to a desired output frequency;e) reducing the temperature and volume of the medium from step c) toobtain the medium in the liquid phase for recycling to step a),wherein the degree of compression in step a) is controlled independentlyof the load applied in step c).

In step a), the medium is thus maintained in the liquid phase, andsubsequently, in step b), it is transformed to a supercritical fluid byheating it above the critical point. The compression of step a) is thusperformed to such an extent that the medium can become supercriticalduring the heating of step b). When the medium becomes supercriticalduring step b), its volume increases. During step c), the pressure drop,which is controlled by the applied load, is recovered as electricalenergy. Thus, the pressure may be dropped to a predetermined level andthe expansion of the medium may be restricted. In practice, step c) isnormally performed by a turbine connected to an electricity generator,which is capable of controlling the load applied in the turbine. In stepe), the volume is decreased. In order to prevent or at least minimize atransition of the medium to the gas phase, it is advantageous to performstep e) as soon as possible after the pressure reduction and energyextraction of step c). In practice, one or more heat exchangers forperforming step e) may therefore be directly connected to an outlet of aturbine performing step c). The compression of step a) is preferablyperformed using a pump driven by motor capable of controlling the degreeof compression of the liquid medium. The pump may thus have afrequency-controlled electric drive.

In an embodiment, the medium is compressed in several, such as two,three or four, stages. In such an embodiment, the medium may be heatedbetween two compression stages. Accordingly, the heating of the mediummay also be performed in several stages with intermediate compressionsof the medium. All such embodiments are encompassed by the presentdisclosure as long as it is a heating and not a compression that finallycauses the transition of the medium to the supercritical state.

Likewise, in an embodiment the pressure reduction is performed inseveral, such as two, three or four stages. In such an embodiment, themedium may be cooled between two pressure reduction stages, e.g. toprevent a transition of the medium to the gas phase. Accordingly, thestep of reducing the pressure and volume of the medium may be performedin several, such as two, three or four, stages with intermediatepressure reductions of the medium.

The method may further comprise sensing the pressure between steps a)and c) and controlling the load of step c) and/or the degree ofcompression of step a) at least partly depending on the sensed pressure.The skilled person understands that “between steps a) and c)” in thiscontext refers to downstream step a), but upstream step c).

Also, the method may further comprise sensing the pressure between stepsc) and a) and controlling the load of step c) and/or the degree ofcompression of step a) at least partly depending on the sensed pressure.The skilled person understands that “between steps c) and a)” in thiscontext refers to downstream step c), but upstream step a).

Preferably, the two examples are combined such that load of step c)and/or the degree of compression of step a) is/are controlled dependingon both the pressure sensed between steps a) and c) and the pressuresensed between steps c) and a).

In an embodiment, the sensed pressure(s) are compared to (a) referencevalue(s) and the load and/or degree of compression is controlledaccording to the result of the comparison. For example, if the pressureof the medium in a position downstream of the turbine is below a firstreference pressure, such as the critical pressure of the medium, theturbine may be controlled to increase the pressure on its downstreamside to at least the first reference pressure. One reason for stoppingthe pressure from falling below the critical pressure is to avoid atransition of the medium to the gas phase. Likewise, if the pressure ofthe medium in a position downstream of the turbine is above a secondreference pressure, the turbine may be controlled to decrease thepressure on its downstream side to a pressure below the second referencevalue by increasing the load, which results in that more electricity maybe generated.

Also, the compression in the pump and the load in the turbine may becontrolled to reach a target pressure for the position upstream of theturbine and the position downstream of the turbine, respectively. Inturn, the target pressures may depend on the amount of external heatingand/or cooling capacity available.

Thus, a sensed pressure between steps a) and c) may be compared to afirst target pressure and the compression and/or load may adjusted toreduce the difference between the sensed pressure and the first targetpressure. Concurrently, the sensed pressure between steps c) and a) maybe compared to a second target pressure and the compression and/or loadmay adjusted to reduce the difference between the sensed pressure andthe second target pressure.

Further parameters of the medium may also be measured and the measuredvalues may be used in the control of the method. For example, thevolumetric flow rate of the medium may be measured with (a) flowmeter(s) between steps a) and b), between steps b) and c), between stepsc) and e) and/or between steps e) and a). Also, the temperature of themedium may be measured with (a) temperature sensor(s) between steps a)and b), between steps b) and c), between steps c) and e) and/or betweensteps e) and a). The measured volumetric flow rate and/or temperaturevalue(s) may also, e.g. after comparisons with reference values, be usedfor controlling the system, e.g. the degree of compression of the mediumin step a), the load in step c), the flow rate/supply of an externalheating medium (from the external heat source) to the heating of step b)and/or the flow rate/supply of a external cooling medium to the coolingof step e).

In prior art energy extraction methods, a liquid medium is oftentransformed to a gas. This transformation requires a lot of energy (dueto the enthalpy of vaporization), which can only be recovered to a smallextent later in the method. According to the method of the presentdisclosure, energy is extracted without any substantial vaporization ofthe medium, and the heat from an external energy source, in particular alow-temperature energy source, may be effectively utilized. Thus, insome embodiments of the invention, the temperature of the externalenergy source is less than 150° C., such as less than 100, 90, 80, 70,60, 50, 40 or 30° C.

In embodiments of the invention, heat recovered from step e) may be usedfor heating the compressed liquid. This may be accomplished in a directand/or indirect manner. That is, heat from the medium from step c) maybe transferred to the compressed liquid from step a) in a heat exchangerand/or a cooling medium circuit may be employed. When the cooling mediumcircuit is employed, step e) comprises heating the cooling medium toobtain a heated cooling medium and step b) comprises heating thecompressed liquid medium using the heated cooling medium to obtain acooled cooling medium which is recycled to step e). Consequently, theheat exchanger and/or the cooling medium circuit is/are part of both theheating of step b) and the cooling of step e).

The use of heat recovered during the step e) for the heating of step b)increases the efficiency of the method.

The cooling medium circuit may involve heat pumping in a conventionalmanner. Consequently, the heated cooling medium from step e) may becompressed before it is used for heating the compressed liquid medium instep b) and the cooled cooling medium from step b) may be expandedbefore it is recycled to step e). If heat pumping is employed in thecooling medium circuit, the internally recovered energy may be suppliedto the heating step as a high-temperature medium, which increases theefficiency of at least some of the embodiments of the inventive method.

In step b), heat from one or more of the following sources may thus besupplied:

-   -   the medium from step c) (direct internal heating);    -   the heated cooling medium (indirect internal heating); and    -   the external heat source.

For example, the compressed medium from step a) may first be heated withinternal heat and then with the external heat. If both direct andindirect internal heating is performed, the compressed medium from stepa) may be heated with the direct internal heat before it is heated withthe indirect internal heat. An example of such a set-up is shown in FIG.3.

However, heat recovered from the cooling step may also be recovered foran external use. For example, step e) may comprise the heating of amedium (e.g. water) for a district heating system. This embodiment isparticularly attractive when the method is performed close to a cityhaving a suitable infrastructure including such a district heatingsystem.

Exhaust gases and industrial cooling medium (e.g. cooling water) areexamples of external heat sources that may be employed in step b), inparticular if the method is performed close to an industrial site orplant. Electricity produced by the method of the present disclosure mayin such case be supplied back to the site or plant. Other examples ofexternal heat sources are a medium (e.g. water) heated by a solarcollector and geothermal heat sources. Yet other examples are groundwater, sea water and fresh water. The external heat source of theinventive method may also be a medium heated by a heat pump utilizingany of the above-mentioned heat sources. This alternative may beparticularly interesting if the available heat source is ground water,sea water or fresh water, since such heat sources may be available inlarge quantities but normally has a relatively low temperature. Groundheat is another example of such a low-temperature heat source. However,the external heat source may also, in some embodiments, be ahigh-temperature heat source, such as an open flame. The open flame maybe provided by combustion of a suitable fuel, such as coke, petroleum,waste or organic material. Thus, the external heat source may be anincineration process. Also, the external heat source may be a radiationsource.

In some embodiments of the inventive method, the temperature of theexternal heat source is at least 5° C. higher, such as at least 10° C.higher, than the critical point for the medium at the prevailingpressure when the heat from the external heat source is supplied.Consequently, the temperature of the external heat source is sufficientfor transforming the compressed medium to the supercritical phase. Thisis however not a requirement; the temperature of the external heatsource may actually be below the critical point in question ifinternally generated heat of a higher temperature (e.g. after heatpumping, see above) is supplied after the external heat.

Various mediums may be used in the inventive method. Examples ofpreferred mediums are carbon dioxide (CO₂), ethylene (C₂H₄), diborane(B₂H₆), ethane (C₂H₆) and nitrous oxide (N₂O). Carbon dioxide isparticularly preferred as it is abundant and has a relatively lowtoxicity. Further, the temperature of the critical point of the CO₂ is31° C. allowing heat sources having a relatively low temperature, suchas 35-125° C. or 45-100° C., to be employed.

The pressure reduction during step c) may for example be controlled soas to balance the pressure in the method. This may be achieved bycontrolling the load of a turbine employed for step c) (see below). Insome embodiments, the method comprises one or more further step(s) ofincreasing or decreasing the pressure for balancing purposes.Consequently, it is not a requirement that the pressure increase of stepa) equals the pressure decrease of step c).

If carbon dioxide is employed, the pressure may for example be increasedby 30-110 bar during step a). Accordingly, the pressure drop during stepc) may for example be 30-110 bar. The inventor has found that somecompression ranges are particularly beneficial from a thermal efficiencystandpoint for some temperatures of the external heat source when themedium is carbon dioxide. Thus, in some embodiments of the inventivemethod, the pressure of the liquid medium is increased by:

35-55 bar, such as 40-50 bar, if the temperature of the external heatsource is 35-65° C.;45-65 bar, such as 50-60 bar if the temperature of the external heatsource is 66-75° C.;55-75 bar, such as 60-70 bar if the temperature of the external heatsource is 76-95° C.; and65-105 bar, such as 70-100 bar if the temperature of the external heatsource is 96-125° C.

In step c), a pressure difference is maintained. Thus, in embodiments ofthe inventive method, a device capable of doing so, such as a positivedisplacement turbine or a reversed centrifugal pump, is employed forstep c).

However, any device which has got a substantially leakage free barrierbetween the inlet and the outlet, and which can be controlled by avariable load, can be used to control the pressure drop to apredetermined level during step c). The barrier can be implemented bydesign, or it can be created as an operational state, at which a barrieris formed. An example of the latter is the reversed centrifugal pump,which utilizes the inertia of the medium to build up barriers betweenthe stator and the rotor, and thus between the inlet and the outlet.

In contrast to the normal behavior of a medium in a traditional turbine,the medium of the present disclosure is preferably not allowed to expandextensively in the turbine. In embodiments of the present disclosure,the density of the medium is thus not decreased or decreased by lessthan 40%, such as less than 30%, such as less than 25%, during step c).

Also, in order to facilitate an efficient internal heat exchange, thetemperature of the medium is preferably not decreased in step c) tobelow a temperature which is 10° C. higher than that of the compressedmedium from step a),

The steps of an embodiment of the inventive method are illustrated inthe phase diagram for carbon dioxide showed in FIG. 4 (see the leftcycle): step a) increases the pressure of the liquid carbon dioxideabove the pressure of the critical point (the temperature is howeverstill too low for the carbon dioxide to transform to the supercriticalstate); step b) increases the temperature of the carbon dioxide abovetemperature of the critical point such that it transforms to asupercritical fluid (and expands); and steps c) and e) reduces thepressure and temperature (and volume) such that the carbon dioxide isobtained as a liquid again.

In an alternative configuration of the present invention, the medium isalways in the supercritical state. The alternative configuration thuscomprises the steps of: a′) compressing the supercritical medium usingan external power source to obtain a compressed liquid medium; b)heating and expanding the compressed supercritical medium from step a′)using heat at least partly derived from the external heat source; c′)reducing the pressure of the heated supercritical medium from step b′)by applying a variable load to generate electric power of a frequency;d′) converting the frequency of step c′) to a desired output frequency;e′) reducing the temperature and volume of the supercritical medium fromstep c′) and recycling it to step a′),

wherein the degree of compression in step a′) is controlledindependently of the load applied in step c′).

The inventor has found that the expansion during the heating of asupercritical medium, such as supercritical carbon dioxide, issufficient for efficiently extracting energy from an energy source. Thefact that a cycle operating within the supercritical state would requirehigher temperatures of the external heat source is however a drawback ofthe alternative configuration in comparison with the invention definedin the appending claims. On the other hand, a process according to thealternative configuration may be less complicated to control and therebycheaper.

The steps of an embodiment of the alternative configuration are alsoillustrated in the phase diagram for carbon dioxide showed in FIG. 4(see the right cycle). It can be seen that the medium (carbon dioxide)is supercritical in each stage of the cycle.

The skilled person understands how to adapt the teachings of the presentdisclosure to the alternative configuration, and the embodiments of thefirst and the second aspect described herein applies mutatis mutandis tothe alternative configuration.

As a second aspect of the present invention, there is thus provided aheat engine system for extracting energy from an external heat source,comprising:

-   -   a pump for compressing a liquid medium to obtain a compressed        liquid medium, said pump comprising an inlet, an outlet and an        external motor capable of controlling the degree of compression        of the liquid medium in the pump;    -   a heating arrangement connected to the external heat source for        heating and expanding the compressed liquid medium to obtain the        medium in the supercritical state, said heating arrangement        comprising an inlet connected to the pump outlet and an outlet;    -   a turbine for generation of mechanical work from the medium from        the heating arrangement, said turbine comprising an inlet        connected to the heating arrangement outlet and an outlet;        a electricity generator connected to the turbine, said        electricity generator being capable of controlling the load of        the turbine such that the pressure upstream of the turbine may        be controlled;    -   a frequency converter connected to the electricity generator;        and    -   a cooling arrangement for reducing the temperature and volume of        the medium, said cooling arrangement comprising an inlet        connected to the turbine outlet and an outlet connected to the        pump inlet,        wherein the compression in the pump is controllable        independently of the load of the turbine.

The pump/compressor is adapted for compressing liquids. The skilledperson is capable of selecting an appropriate device for thecompression. The heating arrangement may comprise one or more heatexchangers. For example, it may comprise at least one heat exchanger fortransferring heat from the medium from the turbine to the medium fromthe pump, at least one heat exchanger for transferring heat from aheated cooling medium to the medium from the compressor and/or at leastone heat exchanger for transferring heat from the external heat sourceto the medium from the compressor. The heat exchangers may be arrangedin any order. It is however preferred that the heat exchanger connectedto the heat source of the lowest temperature is arranged first (furthestupstream) and the heat exchanger connected to the heat source of thehighest temperature is arranged last (furthest downstream). Also, thecooling arrangement normally comprises one or more heat exchangers. Aheat exchanger may in some embodiments be shared by the heatingarrangement and the cooling arrangement.

The cross-sectional area of the channels of the heat exchangersconnected to the outlet of the turbine may be equal to or smaller thanthe cross-sectional area of the outlet of the turbine in order toprevent expansion (and transfer to the gas phase) of the medium.

The various embodiments of the method of the first aspect, and theircorresponding benefits, apply mutatis mutandis to the heat engine systemof the second aspect. However, some embodiments of the heat enginesystem are anyway discussed below.

The heat engine system may further comprise a pressure sensor arrangedfor sensing a pressure of the medium at a position upstream of theturbine and a control device arranged to receive the sensed pressurefrom the pressure sensor and control the load of the turbine and/or thedegree of compression of the medium in the pump at least partlydepending on the sensed pressure. The skilled person understands thatthe “a position upstream of the turbine” in this context refers to aposition upstream of the turbine, but downstream of the pump, such thatthe pressure in the heating arrangement can be determined.

The heat engine system may also further comprise a pressure sensorarranged for sensing a pressure of the medium at a position downstreamof the turbine and a control device arranged to receive the sensedpressure from the pressure sensor and control the load of the turbineand/or the degree of compression of the medium in the pump at leastpartly depending on the sensed pressure. The skilled person understandsthat the “a position downstream of the turbine” in this context refersto a position downstream of the turbine, but upstream of the pump. Thus,the position may for example be between the turbine and the coolingarrangement or between the cooling arrangement and the pump.

The control device receiving the sensed pressure from the positionupstream of the turbine is preferably, but not necessarily, the same asthe control device receiving the sensed pressure form the positiondownstream of the turbine. A common control device enables a moreaccurate and efficient control of the pressures in the whole system andthus provides for higher over-all efficiency in the energy extraction.

The control device(s) is/are normally operatively connected to thepressure sensor(s) and the electricity generator (to control the load)and/or the external motor (to control the degree of compression),preferably via signal lines.

The heat engine system may also further comprise one or more flow metersand/or one or more temperature sensors. Such a meter or sensor may bearranged to measure the volumetric flow rate or temperature of themedium in one or more of the following positions: between the pumpoutlet and the heating arrangement inlet; within the heatingarrangement; between the heating arrangement outlet and the turbineinlet; between the turbine outlet and the cooling arrangement inlet;within the cooling arrangement; and between the cooling arrangementoutlet and the pump inlet.

The flow meter(s) and/or temperature sensor(s) may be operativelyconnected to the control device discussed above or to one or more othercontrol device(s). The control device(s) may thus be arranged to receivethe measured volumetric flow rate(s) and/or temperature(s) and controlthe load of the turbine and/or the degree of compression of the mediumin the pump at least partly depending on the measured volumetric flowrate(s) and/or temperature(s).

Further, control device(s) may also be arranged to control the supply ofa cooling medium to the cooling arrangement and/or the supply of aheating medium to the heating arrangement in response to the input datadiscussed above. Accordingly, the control device(s) may be operativelyconnected to first valve arranged on a cooling medium supply lineconnected to the cooling arrangement and/or a second valve arranged on aheating medium supply line connected to the heating arrangement.

Thus, the control of the system may be even more refined, which allowsfor an even more efficient energy extraction.

An example of a turbine capable of generating mechanical work whilemaintaining the pressure difference between the upstream and thedownstream side of it is a volumetric turbine, such as a positivedisplacement turbine. In the volumetric turbine, the torque may behigher than in many other types of turbines and the speed of the turbinemay be about equal to the volumetric flow rate of the medium. Anotherexample is a reversed centrifugal pump, which utilizes the inertia ofthe medium to build up a pressure behind it.

The turbine is connected to an electricity generator, for examplethrough a shaft of the turbine.

Preferred embodiments of the turbine are disclosed in the patentapplications NO20092085 and WO2008004880, which are incorporated hereinby reference in their entirety. Consequently, the turbine of the presentdisclosure may be defined as in the any one of the claims ofNO20092085/WO2010137992 or WO2008004880.

In order to reduce the expansion of the medium in the turbine, the areaof the outlet of the turbine may be less than 1.5 times the area of theinlet of the turbine. In some embodiments, the outlet area is less than1.3 times, such as less than 1.1 times, the inlet area.

As mentioned above, the load (and thus the energy output) of the turbineis controllable. Consequently, the pressure drop of the turbine may becontrolled by controlling the load, and the pressure in the differentsegments of the system may be balanced. In some embodiments, the systemfurther comprises at least one pressure control device of known type,such as at least one pressure reduction valve, arranged upstream and/ordownstream of the turbine. The purpose of such a device is also tobalance the pressure in the system. However, the skilled personunderstands that the use of such a device may imply a loss inefficiency.

As mentioned above, the heat exchanger arrangement may in someembodiments comprise a heat exchanger connected to the pump outlet andthe turbine outlet such that heat can be transferred from the mediumfrom the turbine to the compressed medium from the pump.

Further, the cooling arrangement may comprise a heat exchanger connectedto a cooling medium circuit. The cooling medium circuit may also beconnected to the heat exchanging arrangement such that the coolingmedium can be used for cooling in the cooling arrangement and heating inthe heating arrangement.

The cooling circuit may further comprise a cooling medium compressor anda cooling medium expansion device, wherein the cooling medium compressoris arranged downstream the cooling arrangement and upstream the heatingarrangement in the cooling circuit and the cooling medium expansiondevice is arranged downstream the heat exchanging device and upstreamthe cooling arrangement in the cooling circuit.

The inventive finding may also be described with the following itemizedembodiments. The various embodiments of the first and second aspectdescribed above as well as their benefits apply to the items belowmutatis mutandis.

1. Method of extracting energy from an external heat source, comprisingthe steps of:a) compressing a medium in the liquid phase to obtain a compressedliquid medium;b) heating the compressed liquid medium from step a) using heat at leastpartly derived from the external heat source to expand the medium andobtain it in the supercritical state;c) reducing the pressure of the heated medium from step b) under theextraction of energy;d) condensing the medium from step c) to obtain the medium in the liquidphase for recycling to step a).2. Method according to item 1, wherein part of the heat supplied in theheating step b) is derived from the condensing step d).3. Method according to item 2, wherein heat from the medium from step c)is transferred to the compressed liquid from step a) in a heatexchanger.4. Method according to any one of the preceding items, wherein step d)comprises heating a cooling medium to obtain a heated cooling medium andstep b) comprises heating the compressed liquid medium using the heatedcooling medium to obtain a cooled cooling medium which is recycled tostep d).5. Method according to item 4, wherein the heated cooling medium fromstep d) is compressed before it is used for heating the compressedliquid medium in step b) and the cooled cooling medium from step b) isexpanded before it is recycled to step d).6. Method according to item 4 or 5, wherein step b) comprises:b1) transferring heat from the medium from step c) to the compressedliquid from step a) in a heat exchanger;b2) heating using heat from the heated cooling medium; andb3) heating using heat from the external heat source.7. Method according to any one of the preceding items, wherein step d)comprises heating water for a district heating system.8. Method according to any one of the preceding items, wherein theexternal heat source is selected from exhaust gases, industrial coolingwater, heated water from a solar collector, geothermal heat sources,ground water, sea water or fresh water.9. Method according to any one of the preceding items, wherein thetemperature of the external heat source is less than 100° C.10. Method according to any one of the preceding items, wherein thetemperature of the external heat source is at least 5° C. higher, suchas at least 10° C. higher, than the critical point for the medium at theprevailing pressure when the heat from the external heat source issupplied.11. Method according to any one of the preceding items, wherein themedium is selected from carbon dioxide (CO₂), ethylene (C₂H₄), diborane(B₂H₆), ethane (C₂H₆) and nitrous oxide (N₂O).12. Method according to anyone of the preceding item, wherein thepressure reduction during step c) is controlled to balance the pressurein the energy extraction method.13. Heat engine system for extracting energy from an external heatsource, comprising:

-   -   a compressor for compressing a liquid medium to obtain a        compressed liquid medium, said compressor comprising an inlet        and an outlet;    -   a heat exchanging arrangement connected to the external heat        source for heating and expanding the compressed liquid medium to        obtain the medium in the supercritical state, said heat        exchanging arrangement comprising an inlet connected to the        compressor outlet and an outlet;    -   a turbine for generation of mechanical work from the medium from        the heat exchanging arrangement while maintaining a pressure        difference between the upstream and the downstream side of it,        said turbine comprising an inlet connected to the heat        exchanging arrangement outlet and an outlet; and    -   a condensing arrangement for condensing the medium from the        turbine to a liquid, said condensing arrangement comprising an        inlet connected to the turbine outlet and an outlet connected to        the compressor inlet.        14. Heat engine system according to item 13, wherein the turbine        is a positive displacement turbine or a reversed centrifugal        pump.        15. Heat engine system according to item 13 or 14, wherein the        turbine comprises a shaft and the electricity generator is        mechanically connected to the turbine shaft.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the drawings, reference number 1 refers to a heat engine system wherea medium in a closed-loop and fluid-tight circuit 2 undergoes atranscritical cycle. The closed-loop and fluid tight circuit 2 isprovided with a pump or compressor 22, a turbine 24, a pipe 26 which isin fluid connection with the compressor outlet 222 and the turbine inlet244, and a pipe 28 that is in fluid connection with the turbine outlet242 and pump inlet 224. The medium flows through circuit 2 in adirection indicated by arrows on the drawings.

In a first embodiment shown in FIG. 1, part of the pipe 26 downstream ofthe pump 22 outlet 222 is provided with a heat exchanger 3. The heatexchanger 3 may have a first side in fluid contact with the circuit 2and a second side arranged to contain a second medium, which may bedifferent from the medium in the closed circuit 2, in heat-exchangingindirect contact with the medium in the circuit 2. Heat exchanger 3 canbe e.g. a counter-current plate heat exchanger of known type, and willnot be discussed further. Heat exchanger 3 can be supplied with energyfrom an energy source 39 through the second medium. The energy source 39may comprise, without being limited to, a boiler for fossil fuels,exhaust gas, cooling water from industry, cooling water from thermalpower plants, hot water from a solar collector, a geothermal source,groundwater, seawater, fresh water from a lake or a river. The energysource 39 may be warmer than the medium in circuit 2. Alternatively, theenergy source 39 may include a medium that is colder than the medium incircuit 2, but which by means of a heat pump of known type (not shown),supplies heat to the medium in the closed circuit 2. The flow directionof the second medium is indicated by arrows.

In a section of the pipe 28 upstream the pump inlet 224, the pipe 28 isprovided with a second heat exchanger 4 having a first side in fluidcontact with the circuit 2. The heat exchanger 4 has a second sidearranged to contain a third medium, which may be different from themedium in circuit 2 and the second medium in heat exchanger 3, inheat-exchanging indirect contact with the medium in circuit 2. Heatexchanger 4 can be e.g. a countercurrent heat exchanger plate of knowntype, and will not be discussed further. Heat exchanger 4 can deliverenergy to an energy sink 49 through the third medium. The energy sink 49may comprise, without being limited to, groundwater, seawater, freshwater lake or river or a heat engine that supplies energy to a districtheating system. The flow direction of the third medium is indicated byarrows.

A second embodiment is shown in FIG. 2. The same items as in theembodiment of FIG. 1 are specified with the same reference numbers andare not discussed further. In a section of the pipe 26 downstream of thepump outlet 222 and upstream of the heat exchanger 3, the pipe 26 isprovided with a third heat exchanger 5. The heat exchanger 5 has a firstside in fluid contact with the pipe 26 and a second side in fluidcontact with pipe 28. The heat exchanger 5 can be e.g. a countercurrentheat exchanger plate of known type, and will not be discussed further.

A third embodiment is shown in FIG. 3. The same items as in theembodiments from FIGS. 1 and 2 are specified with the same referencenumbers and are not discussed further. In a part of pipe 26 downstreamof the third heat exchanger 5 and upstream of the first heat exchanger3, the pipe 26 is provided with a fourth heat exchanger 6. Heatexchanger 6 has a first side in fluid contact with the pipe 26 and asecond side in fluid contact with the second heat exchanger 4. Heatexchanger 6 can be e.g. a countercurrent heat exchanger plate of knowntype, and will not be discussed further. In this embodiment, it isadvantageous if the second heat exchanger 4 and the fourth heatexchanger 6 are parts of a single heat pump 7. The second heat exchanger4 is thus the evaporator 74 of the heat pump 7 and the fourth heatexchanger 6 is thus the condenser 76 of the heat pump 7. As is known inthe art, the heat pump is further provided with a compressor 72 and areducing valve 78. Instead of a reduction valve 78, the heat pump 7 maybe provided with an expander 78′. The heat pump 7 is a closed-loop andfluid tight circuit where the heat pump 7 components are in fluidcontact with closed pipes 71. The medium in the heat pump 7 may be anysuitable medium known in the field, including CO₂. The heat pump 7 maybe of a type shown in the publications WO90/07683, U.S. Pat. No.6,877,340 and U.S. Pat. No. 7,674,097.

A fourth embodiment is shown in FIG. 5. The same items as in theembodiment of FIG. 1 are specified with the same reference numbers andare not discussed further. The pump 22 is driven by an electrical motor32. The motor 32 is capable of controlling the/frequency/speed of thepump and thus the degree of compression of the medium in the pump. Theturbine 2 is connected to a electricity generator 34 via the shaft 35 ofthe turbine 24 such that the electricity generator 34 may control theload applied in the turbine. The electricity generator 34 is connectedto a frequency converter (not shown). Further, a pressure sensor 500 isarranged in the pipe 26 at a position between the outlet 222 of the pump22 and the inlet 244 of the turbine 24 and another pressure sensor 502is arranged in the pipe 28 at a position between the outlet of thecooling arrangement 25 and the inlet 224 of the pump 22. Signal lines501, 503 connects the respective pressure sensors 500, 502 to a controldevice/computer 504 adapted to receive the sensed pressures. The controldevice/computer 504 is connected to the electricity generator 34 and themotor 32 of the pump 22 via signal lines 505, 506. The controldevice/computer 504 is adapted to process the sensed pressures (i.e.pressure values) and send control signals to the electricity generator34 and motor 34, respectively, which control signals are functions ofthe sensed pressures and optionally other parameters. The processing mayfor example comprise comparing the pressure values to reference ortarget values.

The pump or compressor 22 is designed to be able to bring the pressurein the medium in circuit 2 to a pressure above the supercritical limit.Such pumps 22 are known in the art and will not be discussed further.

The turbine 24 can be a differential pressure turbine. Examples of asuitable turbine are described in the applicant's own publicationNO20092085. Differential pressure turbines are liquid tight in thatfluid cannot leak through the turbine housing. The only fluid passingthe house is the volume trapped and transported in the compartmentbetween the vanes when the impellers are rotating. The amount of fluidthat flows through the turbine housing depends on the impeller(s)rotation speed. By slowing the impeller(s), by applying load to theshaft, the amount of fluid that flows through the turbine housing andthe fluid pressure downstream of the impeller can be controlled.Consequently, the pressure can be controlled by controlling the flow offluid through the turbine. A reverse centrifugal pump or a piston pumpof known type are examples that can also be used for this purpose.

The heat engine system will be described in the following examples.Basis for the examples apply the following table values:

Temperature (° C.) 0 60 Pressure (MPa) Pressure (MPa) 3.5 8.0 3.5 8.0Enthalpy (kJ/kg) 200.0 196.5 508.6 458.1 Specific weight (kg/dm³⁾ 0.9280.961 0.064 0.192 Specific volume (dm³/kg) 1.078 1.040 15.66 5.219Viscosity (cP) 0.994 0.109 0.017 0.020

The following examples are simplifications, and a person skilled in theart will be able to complete the calculations and energy balances.

Example 1

The medium used in the heat engine system is CO₂. The example assumesthat the heat engine system is dimensioned for a medium capacity of 100kg/s. The example further assumes that the medium at the pump inlet 224has a temperature of 0° C. and a pressure of 3.5 MPa (corresponding to35 bar). Under these conditions, the CO₂ is in sub-critical phase.

Pump 22 increases pressure in the medium to 8.0 MPa. 100 kg CO₂ at 0° C.and 3.5 MPa results in a volume of 0.1078 m³. The pressure differentialis 4.5 MPa. The work performed by the pump 22 is thus:

100 kg/s×1078 dm³/kg×4.5 MPa=485.1 kW.

Downstream of the pump 22, by the outlet 222, 100 kg CO₂ will constitutea volume of 0.1049 m³. The medium temperature will be 1,488° C. and theenthalpy 200.0 kJ/kg. Under these conditions, the CO₂ is in subcriticalphase.

The pressure in the pipe 26 is maintained at 8.0 MPa as the mass of CO₂that flows through the turbine 24 per unit of time is the same as themass that flows through the pump 22 per unit of time.

To the heat exchanger 3, a work of 25813 kJ/s, or 25,830 kW is supplied.The temperature of the medium increases from 1.488° C. to 60° C. and themedium transforms to the supercritical state under these conditions. Theenthalpy is 458.1 kJ/kg. The enthalpy increase over the heat exchanger 3is thus 458.1 kJ/kg−200.0 kJ/kg=258.1 kJ/kg. The specific volume is5.219 dm³/kg and the medium is in the supercritical state.

The turbine 24 is designed to have a capacity of 0.5219 m³/ssupercritical fluid at a temperature of 60° C. and a pressure of 8.0MPa. The turbine 24 drives a generator (not shown) producing electricalenergy in a known manner. The turbine 24 reduces pressure in the mediumfrom 8.0 MPa to 3.5 MPa. The turbine 24 performs a work of 0.5219m³×4.5=2348 kW.

The turbine 24 can be of a known type, as long as it is designed toperform a controlled pressure reduction between the turbine inlet 244and the turbine outlet 242, where the first pressure is higher than thesecond one.

The example assumes that the turbine 24 is followed immediately by thecooling arrangement 25 provided with the second heat exchanger 4, andthe cooling arrangement 25 is designed to be able to bring thetemperature and pressure of the CO₂ medium to 0° C. and 3.5 MPa,respectively. In the cooling arrangement 25 or the heat exchanger 4,23,483 kW are removed from the medium. The medium is returned to pump 22in this state.

Theoretical net energy gain for a cycle of 100 kg/s medium is: Energyharvested from the turbine 24−energy flow in the pump 22: 2348 kW−485kW=1863 kW. The relationship between the energy supplied in the form ofheat exchanger 3 and the energy harvested in the turbine 24 provides anet theoretical effect of 7.2%.

Example 2

Example 2 assumes the same conditions as in Example 1. The pipe 26routes the medium from the outlet 222 of the pump 22, to the coolingarrangement 25 and through a third heat exchanger 5 which is arrangedimmediately downstream of the outlet 242 of the turbine 24. The mediumwill in a third heat exchanger 5 be heated to 40° C. The pressure is 8.0MPa. Under these conditions, the medium has an enthalpy of 402.9 kJ/kgand a specific volume of 3.599 dm³/kg. The medium is in supercriticalstate.

From the third heat exchanger 5, the medium is routed trough the pipe 26to the heat exchanger 3 where it receives 5523 kW. The medium is therebyheated to 60° C. Then, it flows into the inlet 244 of the turbine 24 insupercritical state at 8.0 MPa and 60° C.

Downstream of the turbine 24 outlet 242, the medium flows through thethird heat exchanger 5 and transfers energy to countercurrent medium asdescribed above. Downstream of the heat exchanger 5, the medium flowsthrough the heat exchanger 4 and then to the cooling arrangement 25.Heat exchanger 4 is designed to bring CO₂ to 0° C. and 3.5 MPa. In theheat exchanger, 43166.7 kW are removed from the medium.

Theoretical net energy gain for a cycle of 100 kg/s medium is: Energyharvested from the turbine 24−energy flow in the pump 22: 2348 kW−485kW=1863 kW. The relationship between the energy supplied in the form ofheat energy recorder 3 and the energy harvested in the turbine 24provides a theoretical net effect of 34.1%.

Example 3

Scenario 3 assumes the same conditions as in example 1 and 2. In thisexample, heat exchanger 4 represents the evaporator 74 of the heat pump7, as shown in FIG. 3.

In Example 3, we assume that the heat pump 7 efficiency is 50%. Therebyheat pump 7 transfers 1583 kW from the evaporator 74 and to the fourthheat exchanger 6. In the heat exchanger 3 downstream of the heatexchanger 6, 3940 kW is supplied to the medium.

Theoretical net energy gain for a cycle of 100 kg/s medium is: Energyharvested from the turbine 24−energy flow in the pump 22: 2348 kW−485kW=1863 kW. The relationship between the energy supplied in the form ofheat supplied to heat exchanger 3 and the energy harvested in turbine 24provides a net theoretical effect of 47.4%.

Example 4

As an illustrative example, a system comprising pipes having an innerdiameter of 100 mm is described. Thus, the cross-section area is 78.5cm² and one meter of the pipe contains 7.85 liters of medium. If 7.85liters per second is pumped, the velocity of the medium will be 1 m/s.

For example, if 50 kg of CO₂ is pumped from a pressure to 73 bar to apressure of 100 bar, the density of the medium will be 0.95 kg/l if thetemperature is 4° C. Thus, the volume will be 5235 l, resulting in avelocity of 6.67 m/s. If the medium is then heated to 100° C., thedensity will be 0.189 kg/l resulting in a speed of 33.76 m/s.

The inlet and the outlet of the turbine have the same cross-sectionarea. Thus, the medium is not allowed to expand to the same degree as inan expander or an expansion turbine. To reduce the pressure back to 73bar between the inlet and the outlet, the work will be 14.31 kJ/kg andthe temperature, density and velocity of the medium will be 74.5° C.,0.150 kg/l and 42.5 m/s, respectively, after the turbine. Heat istransferred from the medium from the turbine outlet to the medium fromthe pump, preferably in a counter-flow heat exchanger, such that thetemperature of the medium is reduced directly downstream the turbine.For example, the pressure may be reduced to 73 bar and a temperature of49.4° C., at which point the density of medium is the same as at 100 barand 100° C. The “internal cooling” of the medium from the turbine isfollowed by “external cooling” such that the temperature of the mediumis reduced below the critical temperature, i.e. below 31° C. Thus, theturbine will be controlled such that the medium reaches a targetpressure (e.g. 73 bar) downstream of it, but the efficiency of thecooling of the medium will determine at which speed the turbine may runto maintain the target pressure provided that enough external energy isavailable for further heating the pressurized medium from the “internal”heat exchange.

1. Method of extracting energy from an external heat source, comprisingthe steps of: a) compressing a medium in the liquid phase using anexternal power source to obtain a compressed liquid medium; b) heatingthe compressed liquid medium from step a) using heat at least partlyderived from the external heat source to expand the medium and obtain itin the supercritical state; c) reducing the pressure of the heatedmedium from step b) to a controlled degree by applying a variable loadto generate electric power of a frequency; d) converting the frequencyof step c) to a desired output frequency; and e) reducing thetemperature and volume of the medium from step c) to obtain the mediumin the liquid phase for recycling to step a), wherein the degree ofcompression in step a) is controlled independently of the load appliedin step c) and step c) is performed by a turbine connected to anelectricity generator. which is capable of controlling the load appliedin the turbine.
 2. Method according to claim 1, further comprisingsensing the pressure between steps a) and c) and controlling the load ofstep c) and/or the degree of compression of step a) at least partlydepending on the sensed pressure.
 3. Method according to claim 1,further comprising sensing the pressure between steps c) and a) andcontrolling the load of step c) and/or the degree of compression of stepa) at least partly depending on the sensed pressure.
 4. Method accordingto claim 1, wherein part of the heat supplied in the heating step b) isderived from step e).
 5. Method according to claim 4, wherein heat fromthe medium in step e) is transferred to the compressed liquid mediumfrom step a) in a heat exchanger.
 6. Method according to claim 1,wherein step e) comprises heating a cooling medium to obtain a heatedcooling medium and step b) comprises heating the compressed liquidmedium using the heated cooling medium to obtain a cooled cooling mediumthat is recycled to step e).
 7. Method according to claim 6, wherein theheated cooling medium from step e) is compressed before it is used forheating the compressed liquid medium in step b) and the cooled coolingmedium from step b) is expanded before it is recycled to step e). 8.Method according to claim 6, wherein step b) comprises: b1) transferringheat from the medium from step c) to the compressed liquid from step a)in a heat exchanger; b2) heating using heat from the heated coolingmedium; and b3) heating using heat from the external heat source. 9.Method according to claim 1, wherein step e) comprises heating water fora district heating system.
 10. Method according to claim 1, wherein theexternal heat source is selected from exhaust gases, industrial coolingmedia, heated media from a solar collector, geothermal heat sources,ground water, sea water and fresh water.
 11. Method according to claim1, wherein the temperature of the external heat source is less than 100°C.
 12. Method according to claim 1, wherein the temperature of theexternal heat source is at least 5° C. higher, such as at least 10° C.higher, than the critical point for the medium at the prevailingpressure when the heat from the external heat source is supplied. 13.Method according to claim 1, wherein the medium is selected from carbondioxide (CO2), ethylene (C2H4), diborane (B2H6), ethane (C2H6) andnitrous oxide (N2O).
 14. Method according to claim 1, wherein thepressure reduction during step c) is controlled to balance the pressurein the energy extraction method.
 15. Method according to claim 1,wherein the density of the medium is not decreased or decreased by lessthan 40%, such as less than 30%, such as less than 25%, during step c).16. Method according to claim 1, wherein temperature is not decreased instep c) to below a temperature which is 10° C. higher than that of thecompressed medium from step a).
 17. Heat engine system for extractingenergy from an external heat source, comprising: a pump for compressinga liquid medium to obtain a compressed liquid medium, said pumpcomprising an inlet, an outlet and an external motor capable ofcontrolling the degree of compression of the liquid medium in the pump;a heating arrangement connected to the external heat source for heatingand expanding the compressed liquid medium to obtain the medium in thesupercritical state, said heating arrangement comprising an inletconnected to the pump outlet and an outlet; a turbine for generation ofmechanical work from the medium from the heating arrangement, saidturbine comprising an inlet connected to the heating arrangement outletand an outlet; a electricity generator connected to the turbine, saidelectricity generator being capable of controlling the load of theturbine such that the pressure upstream of the turbine may becontrolled; a frequency converter connected to the electricitygenerator; and a cooling arrangement for reducing the temperature andvolume of the medium, said cooling arrangement comprising an inletconnected to the turbine outlet and an outlet connected to the pumpinlet, wherein the compression in the pump is controllable independentlyof the load of the turbine.
 18. Heat engine system according to claim17, further comprising a pressure sensor arranged for sensing a pressureof the medium at a position upstream of the turbine and a control devicearranged to receive the sensed pressure from the pressure sensor andcontrol the load of the turbine and/or the degree of compression of themedium in the pump at least partly depending on the sensed pressure. 19.Heat engine system according to claim 17, further comprising a pressuresensor arranged for sensing a pressure of the medium at a positiondownstream of the turbine and a control device arranged to receive thesensed pressure from the pressure sensor and control the load of theturbine and/or the degree of compression of the medium in the pump atleast partly depending on the sensed pressure.
 20. Heat engine systemaccording to claim 17, wherein the turbine is a volumetric turbine or areversed centrifugal pump.
 21. Heat engine system according to claim 17,wherein the turbine comprises a shaft and the electricity generator ismechanically connected to the turbine shaft.
 22. Heat engine systemaccording to claim 17, wherein the area of the outlet of the turbine isless than 1.5 times the area of the inlet of the turbine.